Who are the Seven Sisters? They define San Luis Obispo’s unique character and have been memorialized as the icons of our area in innumerable art forms. They are not women but extinct volcanos. Or, to be more precise, they are volcanic “plugs,” or peaks. These plugs are the remnants of the hardened lava, or plumbing, at the core of a chain of volcanos that were formed 20 to 26 million years ago. The slopes of the volcanoes have since eroded away, leaving the “plugs,” or hardened volcanic rock, exposed. These rugged peaks form the most distinctive feature of the scenery of San Luis Obispo.
The volcanic peaks run through a twelve mile swatch of San Luis Obispo County, starting with Islay Peak in the southern portion of the City of San Luis Obispo; running north as the dividing line between the Chorro Valley to the east and the Los Osos Valley to the west, and continuing on to Morro Rock, ending with the submerged Davidson Seamount about two and one-half miles off the coast of Morro Bay. These volcanic peaks form a part of the daily lives of the residents of San Luis Obispo, Los Osos, and Morro Bay. Prized views of the peaks are shared by many residences and businesses in the local communities.
Just as sisters in a family each have a unique personality while retaining their familial relationship, the Seven Sisters have a similar geological composition but each bear a unique shape and size that makes them readily distinguishable from each other. The legendary Morro Rock is the most famous of the sisters, so named because it resembles the round turban worn by the Moors. Thanks to Morro Rock’s prominent position along the Pacific Coast it has been used as a navigational guide for centuries beginning with the Chumash Indians, and is known in some circles as the Gibralter of the Pacific.
Perhaps due to the stature of Morro Rock, the other sisters in the volcanic chain also came to be known locally as “the morros.” Technically this terminology is inaccurate because only Morro Rock resembles a Moor’s turban. But “morros” is a fitting family name and so much more romantic than “volcanic plug”!
Locals will debate whether there are seven sisters or nine in this family of geological formations. In fact, there may be more than fifteen. For many years, local tradition identified the best known of the morros as the Seven Sisters. In 1964, in an effort spearheaded by historian Louisiana Clayton Dart, county residents succeeded in winning official name recognition from the United States Geological Survey for two additional sisters, leading to the addition of two more sisters to the morro family.
During the 1950s and 60s young local students received instruction on the Seven Sisters. We have adopted the long standing tradition represented by this course of instruction to identify the following morros as the “Seven Sisters.”
Islay Peak: 775 feet; named for a Chumash Indian word meaning “wild cherry.” This peak is located near the San Luis Obispo County Airport.
San Luis Mountain aka Cerro San Luis: 1,292 feet; named for St. Louis, Bishop of Toulouse, France. This peak sits on prominent display over the City of San Luis Obispo and is marked with an “M” for Mission College Preparatory School, which sits near its base.
Bishop’s Peak: 1,559 feet; named because in profile its three sharp points resemble the shape of a bishop’s miter, or hat.
Cerro Romualdo: 1,306 feet; this peak stands over the Huerto de Romualdo Rancho, the only Mexican land grant received by a Chumash Indian in San Luis County. Cerro Romualdo lies within the boundaries of Camp San Luis Obispo.
Hollister Peak: 1,404 feet; originally known as “Cerro Alto,” it was renamed for the pioneer family that owed the ranch at the base of the peak. Cuesta Community College lies between Cerro Romualdo and Hollister Peak.
Morro Rock: 581 feet; first named “El Moro” in 1542 by Juan Rodriguez Cabrillo.
Davidson Seamount: estimated 7,800 feet; first located in 1932 and named for George Davidson, a member of the first California Geological Survey. It lies submerged 3,200 feet below the water’s surface and approximately two and one-half miles off the coast.
Expanding the family of morros to Nine Sisters in 1964, we add:
Chumash Peak: 1,257 feet; named for the Chumash Indians, the original inhabitants of the valleys surrounding the peaks; located between Bishop’s Peak and Cerro Romualdo.
Cerro Cabrillo: 911 feet; named for Portuguese explorer Juan Rodriguez Cabrillo, who sailed under the flag of Spain; located in Morro Bay State Park.
Other known volcanic plugs that round out the family of morros are:
Unnamed Hill. Located just south of Islay Hill.
Righetti Hill aka Mine Hill: 563 feet; located north of Islay Hill overlooking Tank Farm Road.
Orcutt Knob: 569 feet; overlooking Orcutt Road.
Terrace Hill: 501 feet; located adjacent to French Hospital in the City of San Luis Obispo.
Unnamed Hill: 810 feet; located just north of Chumash Peak.
No Name Hill: 1,102 feet; located near Hollister Peak.
Turtle Rock: 209 feet; located just north of Cerro Cabrillo.
Black Hill: 665 feet; located in Morro Bay State Park.
For more information about the fascinating history of these local treasures we recommend Mountains of Fire: San Luis Obispo County’s Nine Sisters – A Chain of Ancient Volcanic Peaks, by Sharon Lewis Dickerson, published by EZ Nature Books (1990). It is available for purchase at the History Center of San Luis Obispo County, 696 Monterey Street, San Luis Obispo, CA 93401; http://historycenterslo.org/. For more technical details, we recommend The Geology of San Luis Obispo County: A Brief Description and Field Guide, by David H. Chipping, Ph.D., California Polytechnic State University San Luis Obispo, published by El Corral Publications (1996). It is available at the San Luis Obispo County Public Library. For a general overview, we recommend the excellent research complied on the Nine Sisters by the Sierra Club, Santa Lucia Chapter, located on the Web at http://santalucia.sierraclub.org/ninesis.html. Feel free to contact Doug today, to learn more about The Seven Sisters.